check below results
var html = "\r\n<html>\r\n<body>\r\n\r\n<p>This is a paragraph.</p>\r\n<p>This is a para" +
"graph.</p>\r\n<p>This is a paragraph.</p>\r\n\r\n</body>\r\n</html>\r\n";
HtmlDocument doc = new HtmlDocument();
doc.LoadHtml(html);
var t1 = doc.DocumentNode.SelectNodes("html");
var t2 = doc.DocumentNode.SelectNodes("body");
var t3 = doc.DocumentNode.SelectNodes("html[1]/body");
var t4 = doc.DocumentNode.SelectNodes("html/body");
var t5 = doc.DocumentNode.SelectNodes("//body");
here
doc.DocumentNode.SelectNodes("body");
is not giving any results because there is no
body node in document node level. but you can use
//body xpath to get node anywhere in the document.